1. Field of the Invention
The invention relates to electro-optical data transmission systems, and more particularly, to such systems operative across a rotating interface.
2. Description of the Prior Art
A particular problem arises in connection with shipboard, rotating, radar antenna systems, especially in surveillance radar equipment where the antenna may be relatively large and heavy (either of the feed and reflector type or of the multi-element planar array type). A surveillance capability usually requires such an antenna to be rotatably mounted about a vertical or nearly vertical axis in order to obtain 360.degree. coverage. To avoid the blockage resulting from various parts of the ship's own superstructure, is has been necessary to mount the large and heavy antenna array and its drive mechanism as high as possible, even at the very top of a main mast.
In view of the inherent difficulties, associated with placement of a relatively heavy rotating antenna assembly at the top of the mast, it has been more recently suggested that such an antenna assembly might be arranged on a drive which causes it to rotate about a circle outside of the mast, the array normal usually remaining perpendicular to a tangent of the said circle. Stated otherwise, the array normal always coincides with a line containing a radius of the circle. To accommodate radio frequency power exchange between fixed transmitting/receiving components and such an array, various forms of rotating feeds have been suggested. One particularly advantageous feed for the purpose is described in U.S. patent application Ser. No. 040,325, filed May 18, 1979, and entitled "Around-the-Mast Rotary Coupler", now U.S. Pat. No. 4,222,055, assigned to the assignee of the present invention. A variation of this RF feed arrangement is described in U.S. patent application Ser. No. 107,397, filed Dec. 26, 1979, now U.S. Pat. No. 4,253,101, entitled "Power Transfer Ripple Reduction Method and Means for Rotary Annular Loop RF Coupler," also assigned to the assignee of the present invention. Those inventions deal with the problem of RF energy transfer across a rotating interface, whereas the invention herein described deals with control or information signal transfer, preferably in digital code form.
In addition to radio frequency energy transfer, it is frequently necessary to exchange other signals between the rotating antenna structure and fixed transmit/receive components. One particular requirement for additional signal transfer concerns digital signals for the control of digital phase shifters associated with discrete elements of a planar array, or with discrete rows or columns of elements in such a planar array. The most obvious and basic apparatus for effecting such a transfer would be a slip-ring and brush arrangement, or some form of inductive or capacitative coupling between mutually rotatable transfer structures. The disadvantages of slip-ring and brush arrangements are well known, and inductive and capacitative couplers are difficult to implement satisfactorily, particularly when the radius of the circle of array travel about a mast is relatively large.
The manner in which the present invention uniquely solves the aforementioned signal transfer problem will be understood as this description proceeds.